Method of coating paper



Jan. 2L 1941. B BRAD V 2,229,621

METHOD OF COATING PAPER Filed June 10, 1938 Patented Jan. 21, 1941 units est slppllcsticn dune it, 119%, denied Wee 2123?? n Qiaims.

This invention relates to methods of making one paper sud particularly for making coated paper on a paper mschine.

it has been the almost universal practice in the manufacture of coated paper to make the paper raw stools on "a paper machine, wind it into rolls, and then as a separate operation to apply the coating composition such as, for example, clay, calcium carbonate, hlenc fixe, satin white, end the like sud adhesive such as, for example, casein, starch, etc, in aqueous suspension to one cr'both surfaces of the web by means of rolls, brushes, etc. Such coated paper is usually dried on iestcons and then supercalendered. For s. great many years attempts have been made to coat the paper while it was still on the paper machine, thereby affecting a saving in time, labor, and capital investment. So far as I am aware, however, none of these processes has given a product which ranked in quality with coated pa.- per made by the two-step process.

l have succeeded in making coated paper directly on the paper machine, of s quality closely approximating, if not equalling, the quality of coated paper made on a coating machine from the same grade of raw stock and coating composition and festooning in the commonly practiced method. In some grades the product is indistinguishable from the usual coated paper, and in some respects, as for example printing qualities, the product is frequently superior to a corresponding grade of the usual coated paper.

Briefly, the process consists in applying excess coating composition to one side of the web of paper under conditions adapted to form a. filter cake on the paper, and removing by a wiping action the more fluid coating above the filter cake and thereafter drying the paper preferably on internally heated drying cylinders.

I have discovered that upon the application of a. suitable coating composition to the paper web in its p'a'ssagethrough the paper machine a filter cake is formed of the coating composition immediately adjacent the surface of paper web by the absorption by the paper of a portion of the water in the coating composition, while above the filter cake the liquid content of the composition is not substantially reduced, whereby the layer of coating composition is in effect converted into two layers of different liquid content, so that this outer'layer of higher liquid content may be readily removed by simple apparatus easily applicable to the paper machine, leaving .on the paper a smooth, uniform layer of coating composition of such character that it can be dried by contact with the steam heated cylindrical are com monly used in paper machines without sticking to or being marred by said dryers.

The specific method of coating the web, which I advantageously use, consists of two steps. An aqueous coating composition, the characteristics of which will be more fully discussed later, is first applied all across one surface of the web. The quantity applied is in considerable excess of the weight finally desired on the paper. Advantageously the coating is applied by means of a roll. No particular effort is made at this point to secure a uniform lay of coating, but it is important that all portions of the side being treated come in contact with the coating. The object 35 here is to thoroughly cover the paper with excess coating composition over its entire area.

By a. suitable selection of coating composition, paper stock, time of contact, etc., there is produced on the paper a firm filter cake comprising some of the solid matter in the coating. That is to say, part of the water is absorbed into the sheet, leaving upon the surface of the sheet a substantially continuous layer of filter cake of the desired depth. Above the filter cake is a layer of more fluid coating. The second step consists in removing this fluid layer.

The paper stock is preferably of such a nature as to readily absorb water from the coating composition. A well formed, fairly free slack sized stock such as book paper stock is satisfactory. Moderately wet paper often forms filter cake more readily than completely dry paper, and filter cake can be produced even on paper that has passed through the usual presses of a paper machine before entering the dryers, but it fails to be produced when the paper is so wet that the pores are filled with water. The quantity of filter cake formed depends not only on the raw stock, but on the coating composition, the time of contact before the excess coating is removed, and on the temperature of the raw stock and coating. In the coating composition, different pigments, different adhesives, the presence of dispersing or wetting agents, and, of course, the per cent of water, all influence the rate of filter cake formation. Obviously these factors are all mutually interrelated as regards filter cake formation. Thus lower solids content of a coating may be compensated for by longer time of contact, etc.

The thickness of the filter cake is not significantly affected by the amount of coating applied to the surface. It is essential that there be excess coating over the entire surface, but it is not essential that the quantity of excess be'uniform for the reason that the filter cake is formed by absorption of the water into the paper and the rate of absorption of the water from the coating composition by the paper stock is substantially uniform across the width of the web. Consequently, variations in the depth of the applied coating results in a greater or less excess to be removed. but has no significant effect on the weight of the filter cake.

The removal of the fluid coating from above the layer of filter cake on the paper may be accomplished by a wiping action. Squeeze rolls, due to their tendency to form stipples, are not satisfactory. One device which is particularly suitable for removing the fluid coating layer may take the form of a small radius blade or rod mounted on a rigid support, such as that de scribed in Patent No. 1,944,835 to C. F. Boyers. In the present case the rounded blade rides on the layer of filter cake, scraping off the fluid coating which is returned for reuse, and leaving a firm, substantially continuous layer of coating having a remarkably smooth and uniform surface and completely covering the fibres.

The coating mechanism for coating either side of the web may be installed on the paper machine at the wet end between the press section and the dryers, or it may be installed at different locations in the dryer section. If installed at the wet end of the machine it is preferable to apply coating composition to the wire side only. When the coating composition is applied in suflicient quantity to form a substantially continuous layer of coating over the surface of the web on the felt side at this point, the .urface of the resulting coating is generally marred by a great number of small holes in the coating, varying in size from those visible under a hand lens to those which are easily visible to the naked eye. These holes make the paper unsuitable for high grade printing. but do not prohibit its use for other purposes.

I have found that the ultimate cause of these holes is due to bubbles of air in the stock on the paper machine wire. There are usually some of the bubbles on the wire side of the web, but an enormously greater number on the felt side. In passing through the presses many of these air bubbles may be replaced by water droplets, but whether theybe air bubbles or water droplets the effect is equally disastrous when coating composition is applied over them. The coating does not fill in these spaces and the result is that the dried paper is defective.

When the coating composition is applied to the dried paper as is ordinarily done by means of the usual coating machines, capillary action draws the coating into these spaces or pores and no difliculty is experienced in obtaining a coated paper that is substantially free from these holes. It is not necessary, however, to entirely dry the paper before applying the coating composition. I have found that ordinarily the water droplets and air bubbles above referred to have disappeared after the paper has passed part way through the dryers. The exact point at which this occurs will depend upon a number of conditions such as the number and degree of pressings, paper machine speed, nature of the stock, etc.

In so far as the elimination of these holes is concerned the application of the coating may be carried out at any point beyond the place where the air bubbles and water droplets disappear. However, as the drying operation on the paper machine continues the paper undergoes a decided the moisture content is reduced below a given amount, which differs with different papers, there occurs decided shrinkage and this continues until the paper is thoroughly dry.

The edges of the web of paper usually dry faster than the center portions of the web. Consequently there comes a time in the drying operation when the edges undergo considerable shrinkage while the middle part of the paper has not shrunk as much. Beyond this point, until the paper is dry, there is a definite "edge eifectthe paper tends to be longer in the middle than at the edges and stresses vary across the sheet as a result.

In the drying zone between the place where air bubbles and water droplets disappear and the point where edge effects become noticeable, the coating composition may be advantageously applied to either side of the web. If the coating is applied in advance of the first named location the felt side of the paper is very likely to contain the previously described holes. If applied at or b'eyond the place where the edge effects set in, unequal swelling occurs and the paper may be drawn into large wrinkles and puckers.

I have also found that the coating composition can be applied after a partial shrinkage of the paper provided steps are takento equalize the length of the web all across its face. This may be accomplished by passing the web partly around two suction rolls, the roll with which the paper first contacts traveling somewhat slower than the other roll. By proper regulation of the rela tive speeds of the two suction rolls it is possible to lengthen the edges sufiiciently to enable the coating operation to be successfully carried out. A more simple method is to pass the paper between a pair of press rolls just prior to the coating operation. Also, edge effect may be avoided by trimming off the shrunken edges of the web before applying the coating composition.

Coating mixtures in the state of fluidity in which they are applied to the web will stick to any solid surface with which they come in contact. In other words, such paper coating compositions are relatively low in cohesive properties, but high in adhesive properties. The filter cake produced as herein described, however, is relatively cohesive; it is firm enough within a short time that it can contact a solid surface without being marred. It is the removal of the fluid layer of coating lying above the filter cake, combined with the quick setting of the filter cake that is largely responsible for the success of this method over all previous proposals to coat paper on the paper machine, because it is thereby possible to run the paper through the regular dryers of the paper machine without the coating sticking to the dryers and fouling them.

The invention will be further described by referring to the accompanying drawing in which:

Fig. 1 represents more or less diagrammatically one form of apparatus for carrying out the invention;

Fig. 2 represents diagrammatically an alternative form of apparatus; and v Fig. 3 is a diagrammatic sectional view of a sheet of paper showing the formation of the filter cake and the removal of the excess coating.

The coating mechanism for the wire side may be located in advance of the dryer section of preferably installed at a point somewhat near the middle of the dryer section, beyond the point where air bubbles and water droplets give trouble, but before the edge effect has become pronounced.

The paper web is indicated in Fig. 1 by the numeral I, while}, 2 indicate the final'press rolls of the press section of the paper machine, and 3, 4, 5, 6, 1 represent five of the ordinary steamheated dryer cylinders which make up the dryer section of-the paper machine. A carrier roll 8 is provided in advance of the press rolls for guiding the traveling web of paper. Coating apply= ing roll 9 is driven by a motor, not shown, advantageously inv the opposite direction to the travel of the paper at a speed about equal to that of the paper. Roll 9 dips into the coating composition II in the pan l0. Coating composi- .tion in excess of the amount carried away by the web is supplied to the middle of pan III by means 01 the pipe or hose 1! from a reservoir, not shown, by a pump l6. An overflow i3 is provided at each end of the pan iii. A circulation of coating is thus maintained at all times.

,The coating applying roll 9 applies the coating composition across the entire face of the web in a layer considerably thicker than desired on the finished paper. The coating composition is usually an aqueous suspension of finely divided mineral filler with an adhesive such as casein, or starch, and as the composition is applied by the roller 9 the water in the composition begins to be absorbed by the paper web and by the time the web reaches the scraper ll, Fig. 1, a substantial percentage of the water in the coating composition adjacent the paper web has been absorbed by the paper, thereby forming a filter cake of the desired thickness, with the surplus coating composition forming a more fluid overlying layer. The scraper, as here shown, preferably comprises a small chromium plated, rounded blade ll of'about 1 inch radius mounted on a rigid beam l5 and extending the full width of the paper. Blade I! may be adjusted toward and from roll 9. This blade scrapes oil. the surplus fluid coating composition above the layer of filter cake formed on the paper. The excess coating composition flows back into the pan H). The blade also levels out the inequalities of the coating and leaves it a remarkably uniform layer substantially free from defects of any kind. The

coated paper now passes around the steam heated dryer cylinder 3 with the freshly coated side out of contact with the dryer. This dryer is a standard dryer cylinder, and the travel of the paper around it as shown is sumcient to permit the coated side to contact with dryer cylinder I with- 'out any appreciable sticking of the coating to that dryer.

As stated above the coating mechanism may be installed immediately in advance of the dryer section especially if the coating is applied to the wire side of the paper at this location. The coating mechanism may also be installed inthe dryer section of the machine. If located after edge effects set in means are provided to equalize the shrinkage, as shown for example in Fig. 2.

- The web in Fig. 2 passes around the drying cylinder 22, and then through a pair of press rolls I8 and I9. These rolls are rubber covered and are provided with conventional pressure adlusting means to increase or decrease the pressure between the rolls in the press. These press rolls tend to eliminate the previously mentioned "edge effect. The desired edge stretching eifect may also be obtained by passing the web around two suction rolls running at slightly different speeds.

From the press rolls the web passes around carrier rolls 8 to the applicator roll 8, which dips into the coating composition II in pan 20 similar to that described in Fig. 1.

Instead of using a stationary blade to wipe oi! the coating. a reversely turning roll 24 equipped with a wiper 2| may be used. Roll 24 is driven by a motor, not shown, in a direction opposite to that of the paper. It is also advantageously arranged to dip into coating II in pan 20 so that it will be evenly wetted and thoroughly wiped. This roll wipes oil! the fluid coating above the filter cake. The fluid coating composition thus removed is returned to the pan 20 and thence to the reservoir through the pipe l3. The distance between roll 9 and roll 2'4 may also be adjustable if desired. The web then passes around drying cylinder i1 and its freshly coated side out, and thence through the remaining dryers 25 some of which are shown.

In carrying out this invention it is essential 25 impossible to adjust the spacing between such 30 elements or maintain it sumciently uniform across the width of a paper machine so as to remove essentially all of the fluid coating above the filter cake without digging into and destroying the filter cake in spots. Likewise, lumps of all sorts, such as fibre aggregates, cracked edges, etc., cause frequent breaking down of the web if such closely spaced elements are used.

Fig. 1 shows the paper being coated on the wire side whereas in Fig. 2 the paper is coated on the felt side. It is obvious that the device shown in Fig. 2 could also be used for coating the wire side and vice versa, and that many combinations of press rolls, suction rolls, blades, wiped rolls, etc., may be used. If desired both sides of the web may be coated in succession with or without a drying operation between each application.

It will also be noted that after the coating operation the paper passes directly onto the dryers without the interposition of brushes, squeeze rolls, or any smoothing or coating reapplying apparatus. Such devices are neither necessary nor desirable and may do great harm in disturbing the smoothness or firmness of the coating.

Fig. 3 shows diagrammatically what probably takes place during the formation of the filter cake and the removal of the excess coating composition. As here shown, it indicates the paper web with its rough and porous surface. Upon the application to this surface of the coating composition there is a rapid absorption of water from the applied coating immediately adjacent the paper web, forming a layer of filter cake as illustrated at b. The absorption of the water by the paper is much more rapid than the equalization of the water content in the coating layer throughout its total thickness and hence the liquid content of the coating composition in the outer portion of the applied layer is altered but little for an apprecable time, hence the coating in excess of that necessary to form the filter cake forms an outer layer 0 overlying the filter cake. The line of demarcation between the filter cake and the fluid composition does not appear to be sharp, and between the two layers 11 and 0 there appears to surface of the coating left on the paper and also probably removes some of the layer d, since on continued running there is a small increase in the solids content of the coating composition Ii (Fig. 1). However, it must be kept in mind that the total thickness of the filter cake plus layer d is less than a thousandth of an inch and consequently the composition of layer d is speculative. Regardless of theory, however, a remarkably uniform coated paper is produced by my method.

It is usually satisfactory to coat the wire side of the sheet ahead of the dryer section and to coat the felt side of the paper in the dryer section after the place where air bubbles and water droplets have disappeared. In this way a coated-twosides paper of quality equal to that of coated paper made in .the regular way may be produced directly on the paper machine. If coated-oneside P per is desired, as, for example, label paper, a preliminary coating may be applied ahead of the dryer section and followed if desired. by a superposed coating applied in the dryer section. Likewise. of course, superposed coatings may be applied to both sides of the web.

The following examples are cited to illustrate the invention:

Example 1 A web of paper was formed on a usual Fourdrinier machine from the following furnish:

further Pounds Bleached kraft pulp 350 Bleached soda pulp 1000 Deinked magazine papers 500 Clay filler 600 It contained no rosin sizing or alum. Basis weight of the dry raw stock was 42 pounds per ream of 500 sheets x 38". The paper machine was driven at a speed of 275 feet per minute. After passing through the press section of the paper machine and before entering the dryer section coating material having the following The coating composition had a viscosity of 11 seconds determined by the Stormer method. The coating applying roll was about 6 inches in diameter and was rotated at 300 feet per minute in a direction opposite to the travel of the web. An excess of the coating composition was applied and the scraper, a 1*; inch radius chromium plated blade, was positioned about 8 inches from the applying roll, thereby leaving a layer of coating of 8 pounds dry weight per ream on the wire side of the paper. After passing'around eleven 4-foot diameter dryers the edges of the web had begun to shrink a little more thanthe middle. The web then passed between a pair of rubber covered press rolls after which the felt side was contacted with a similar coating applying roll traveling at a speed of 300 feet per min ute in a direction opposite to that of the travel of the paper. The coating material here applied was of the followiing composition:

It will be noted that this coating differed from that applied on the wire side chiefly in having a slightly higher solids content. The web then passed over a similar scraper blade spaced about 8 inches from the applicator roll. This blade smoothed and levelled the coating and left 7 pounds dry weight of coating per ream upon the felt side of the sheet in a very uniform layer. The paper then passed around the remaining dryers in the paper machine and was supercalende'red. The speed of the paper, viz. 275 feet per minute, was the same as the speed at which this machine normally ran on making a 57-pound basis weight uncoated book paper. The final product was a coated paper having a fine, bright color with a smooth, even surface of good ink receptivity and which gave excellent printing results with fine half tones and with uniform solids either in black or colors. It was fully equivalent to coated paper of a similar grade and weight to which the coating was applied by the usual two-step coating operation.

mple 2 A well formed, unsized, clay filled, book paper stock of basis weight 37 pounds (25 x 38-500 sheets) was formed on a Fourdrinier paper machine at 300 feet per minute. Between the press section and the dryers the wire side of the web was coated with a heavy application of aqueous coating composition in the manner above described. Spaced about 8 inches beyond the roll was a small radius blade which contacted the traveling web of heavily coated paper. In the short distance between the application of the coating composition and contact with the blade a filter cake of the desired thickness was formed upon the paper and the blade riding upon this filter cake wiped off the fluid coating lying above it and returned it for reuse. The coating composition as applied consisted of 1,000 pounds of standard quality coating clay and 120 pounds of casein (dissolved in sodium carbonate solution) and water to give 40% solids. The paper then passed into the dryer section of the paper machine, and about half way through the dryer section the felt side was processed in a similar manner with the same coating composition. The quantity of coating (dry basis, per ream) applied to the wire side was 6 to 7 pounds and the weight applied to the felt side was 6 pounds.

The final product was a coated-two-sides paper of 50 pounds basis weight which could be used directly off the paper machine with no further processing. In this condition it gave definitely superior printing results to 50-pound uncoated paper made from the same furnish at the same paper machine speed and which in addition had been supercalendered. The product above described, after supercalendering, compared favorably with coated paper of similar weight and grade made by the two-step process.

It is not to be understood that the invention of this application is in any way limited to. the making of coated paper for use only in the fields where coated paper produced by the present-day two-step process is used. The process is also adaptable to the making of coated paper for use in many of the fields in which uncoated paper is now principally employed. By the process of this application a coated paper, that is, a paper having on one or both sides a continuous coating composition of such character as to give equal appearance and printing-quality to coated paper made by the present-day processes can be .produced at a cost no greater than high grade uncoated paper of the same basis weight. Increasing the speed of the paper machine adversely affects the printing qualities of thisvpaper, as is also the case with unooated paper. However, by employing the process of the present invention, the printing qualities of the paper are so improved that a coated paper can be produced at a speed of, say, 400 feet per minute, which is superior in printing qualities to an uncoated scribed, it may also be employed for coating prefabricated paper as it leaves the paper machine by the simple expedient of running the web through the coating apparatus and then through an additional bank of drying cylinders. Also, of course, the paper may be wound into rolls in the usual manner and then subsequently coated by my new process and dried on internally heated drying cylinders as a separate operation. Nor is the process necessarily limited to the use of aqueous coating compositions; non-aqueous coating compositions may also be used.

No hard and fast rule can be laid down for making up coating compositions to be used in the practice of the present invention. Generally speaking, it is desirable to lower the water content of the coating composition as far as possible and still leave it sufliciently fluid to be handled by the applying apparatus. The above examples give the proper amounts of water for compositions of the ingredients specified. The amount of water required is not critical, as variations in the amount of water in the composition may be compensated for by increasing or decreasing the time permitted for the formation of the filter cake. In the apparatus shown in the drawing this increase and decrease in the time may be very readily effected by adjusting the scraper blade or wiping roll toward or away from the compositionapplying roller. Other conditions being equal the more water present in the mixture the greater the distance from the applicator, the blade or other wiper should be set.

With the understanding that a firm filter cake of the desired weight is essential, those persons skilled in the paper coating art, particularly in view of the'examples given herein, should have no difilculty in obtaining a suitable combination of raw stock and coating composition to give the desired results.

As has been pointed out above, a variety of means may be employed for removing the fluid coating above thefllter cake in accordance with the present invention, it being essential, however,

that the removal be effected by an action analogous to a "wiping action as distinguished from a squeezing action, whereby the preformed filter cake will not be altered.

I claim: 1. Process of making coated paper which comprises applying an excess of fluent coating composition, comprising a liquid vehicle containing,

pigment in suspension and adhesive, to a travel: ling web of paper which is absorptive to the liquid of the coating composition to a degree to form on the surface of the web a layer of firm filter cake from the pigment in the coating composition adjacent the web while the overlying coating composition remains fluent, then after the formation of the layer of filter cake wiping ofi" substantially all said fluent coating composition without removing substantially any of the filter cake and thereafter drying the coated paper.

2. Process of making coated paper which comprises applying an excess of fluent coatingoomposition, comprising a liquid vehicle containing pigment in aqueous suspension and adhesive, to a travelling web of paper which is absorptive to the liquidof the coating composition to a degree to form on the surface of the web a layer of firm filter cake from the pigment in the coating com-' position adjacent the web while the overlying coating composition remains ,fiuent, then after the formation of the layer of filter cake wiping or substantially all said fluent coating composition without removing substantially any of the filter cake, and thereafter contacting the wiped surface of the coating with the surface of a heated dryer.

3. The process of claim 1 in which the wiping is accomplished by means of a small radius blade operating 'on the unbacked paper web.

4. Process of making coated paper which com prises forming a substantially solid layer of coating composition by applying an excess of a fluent coating composition comprising solids in liquid suspension to a travelling web of paper absorptive to the liquid of the composition, maintaining the face of the traveling web against disturbance to allow a firm filter cake to form on .the paper by the absorption of liquid of the coating composi-- tion, wiping off substantially all the fluent coating down to the filter cake, and then drying the coated paper.

5. Process of making coated paper which comprises forming a substantially solid layer of coating composition by applying an excess of a fluent coating composition comprising solids in liquid suspension to a travelling web of paper absorptive to the liquid of the composition, maintaining the face of the traveling web against disturbance to allow a firm filter cake to form on the paper by the absorption of liquid of the coating composition wiping off substantially all the fluent coating down to the filter cake, and thereafter contacting the wiped surface of the filter cake with a heated drying cylinder.

6. The process of claim 5 in which the wiping is accomplished by means of a. small radius blade operating on the unbacked paper web.

7. Process of making coated paper'which comprises applying an excess of liquid coating-composition containing adhesive and pigment in aqueous suspension to a travelling web of paper sufilciently absorptive to' liquid of the coating composition to form on the web a layer of firm filter cake of progressively increasing thickness from the solids of the coating composition adjacent the web while the overlylng coating compositlon remains liquid, then at a distance from the location of the coating applying means where the layer of filter cake formed contains the weight of coating desired, wiping of! said liquid coating composition at approximately the surface of the filter cake, and thereafter drying the coated web with the wiped filter cake as its exposed surface.

8. The process of making coated paper in a paper machine which comprises applying an excess of liquid coating composition containing adhesive and pigment in aqueous suspension to a partially dewatered travelling web of paper, at a location in-the paper machine where it is sufllciently absorptive to liquid of the coating composition to form on the web a firm filter cake from the solids of the coating composition adjacent the web while the overlying coating composition remains liquid, then wiping off said liquid coating composition approximately at the surface of the filter cake and drying the coated web in the paper machine with the wiped surface of the filter cake contacting heated dryers.

9. The process of making coated paper in a paper machine which comprises applying an excess of liquid coating composition containing adhesive and pigment in aqueous suspension to a partially dewatered traveling web of paper at a location in the paper machine where it is sufiiciently absorptive to liquid of the coating composition to form on the web a firm filter cake from the solids of the coating composition ad- Jacent the web while the overlying coating composition at its outer surface remains liquid, then wiping off said liquid coating composition ap proximately at the surface of the filter cake and thereafter passing the web with the wiped filter cake forming its exposed surface through a drying section of the paper machine consisting of the conventional internally heated drying cylinders.

10. The process of claim 8 wherein the wiping is accomplished by means of a small radius blade operating on an unbacked paper web.

11. The process of claim 8 wherein the wiping is accomplished by means of a wiped roll rotating opposite the direction of travel of the paper and operating on an unbacked paper web.

DONALD B. BRADNER. 

